Optically retrievable information storage systems have been commercially available for some time in the form of video discs and audio discs (more commonly referred to as compact discs, i.e. CD's.) More recently, systems in other forms such as optical tape (Gelbart U.S. Pat. No. 4,567,585), and data information cards, like that developed by Drexler Technology Corporation, Mountain View, Calif. (Drexler U.S. Pat. No. 4,544,835) are beginning to attract commercial attention. Information carriers or storage media such as video discs and audio discs are often referred to as Read-Only Memories (ROM). The information is typically stored as extremely small structural relief features which are permanently molded into the substrate during the manufacturing process. Optical retrieval of such data is typically accomplished through differential reflection techniques using a frequency modulated laser light source.
Information can be placed on these optical memories in extremely high densities, the theoretical limit being determined by the absolute resolving power of a laser beam focused down to its diffraction limited size (.lambda./2NA, wherein .lambda. is the wavelength of the laser and NA is the numerical aperture of the focused beam). The information stored on these ROM media is, in principle, capable of being optically accessed an infinite number of times, and then subsequently electronically decoded, and presented in a format which is meaningful to the user.
In optical storage systems designed for Read-Only applications, the information is commonly stored on the media in the form of extremely small pits and/or protrusions (relief structures), which are present on a highly reflective background layer on the media. As the media is moved relative to the laser beam, the differences in the reflected light signal due to the presence or absence of these pits and/or protrusions are optically detected; thereby providing the coded signals which may then be electronically decoded. The nature of the detected signal modulation from the encoded structures in these systems is typically due to changes in the phase relationship between the reflected light off the relief structure and that off of the surrounding media. For maximum optical definition of the reflected signal during optical reading of the ROM media, the physical distance between the surface of the information carrying spots and the surface of the surrounding medium should be such that the laser light being reflected off the interface between these surfaces will be approximately 180.degree. out of phase with respect to each other. Destructive interference of the read beam results when the imaged laser spot scans over regions defined by the edges of the information spots; resulting in a decrease in the intensity of the reflected beam.
Typically, these relief structures are borne on grooves and/or ridges which provide tracking information to the servo system of the laser reading apparatus. These tracks accurately guide and focus the laser beam over the spots and thus ensure maximum signal contrast between data points. Generally, the pits and/or protrusions are of uniform width and depth but are often of varying lengths along a defined track. These marks can thus comprise a spatial representation of the temporal variation of a frequency modulated carrier signal for video applications, or alternatively represent a more complex form of digital data encoding.
Read-Only-Memory media such as discs are typically fabricated by compression/injection molding of these pits and/or protrusions and/or tracks into the surface of a substrate material, most commonly of a polymeric composition; or alternatively by a photopolymer process in which a photosensitive lacquer is coated directly onto the substrate. The former requires critical control of temperature and pressure cycles to minimize both the deformation of the disc and inclusion of optical nonuniformities (stress birefringence). These two induced defects are particularly critical for systems that are read through the substrate.
The thus molded surface of the ROM substrate is typically coated with a thin layer of a highly reflective material such as aluminum to provide for a reflective background upon which the incident laser beam can discriminate between the digital signals associated with the pits and/or protrusions. A suitable protective coating is then often applied onto the encoded reflective surface to protect this surface from both mechanical damage and environmental degradation, along with ensuring that any dust contamination or surface scratches will be sufficiently out of focus for those systems in which the laser reading is not done through the substrate.
In addition to ROM media, both Write-Once media and Write-Read-Erase systems have been recently introduced into the marketplace. Typically, these systems utilize a diode laser to both "read" and "write" coded information from and to the media. Data can be of several forms: that which includes some permanent pre-recorded data (similar to ROM) in addition to that which can be permanently formed by the laser through direct or indirect interaction by the user (Write-Once); that in which all the information is recorded by the laser: or that which can be interactively formed and removed by the laser (Write-Read-Erase).
Write-Once applications for optical information storage are often referred to as "direct-read-after-write" (DRAW) or more recently, "write-once-read-many" WORM) media. In this application, the optical storage media or disc is typically already preformatted with the appropriate tracking and associated access information. Some of the media incorporates suitably reflective and active layer into the multilayered structure.
Write-Once media which supports recording of audio or video signals are called recordable compact disc memory media. In order for media to be recordable on standard equipment under present industry guidelines, such media must have a reflectivity level of about 70% and at the same time be sensitive enough to be written upon at a rate of about 4.3 MHz, which is the sampling rate required for audio/video signal reproduction. At this sampling rate, the sensitivity of the media should be of sufficient degree to allow writability utilizing about a 10 mW laser. To date, no media has been introduced into the marketplace which can meet these criteria. One embodiment of the present invention does possess the capability for recording at about 4.3 MHz using about a 10w laser at about a 70% reflectivity level.
In the present invention, the reflective and active layer are the same layer, i.e. the alloy. The active layer serves as the medium in which new data can be encoded through interaction with light. For example, a modulated laser beam operates to encode information into this surface by tightly focusing it down to a spot size in which the power per unit area ratio is sufficient to cause a detectable change in the optical characteristics of the active layer on the disc The nature of the light signal modulation detected during the reading process is typically due to differences in the absorption-reflection coefficients of the material; phase relationships of reflected light; and/or polarization states of the reflected beam.
Typical forms of WORM media (typically grouped together by the mechanism in which the encoding of data is accomplished) include: phase change systems (Blonder et al. U.S. Pat. No. 4,579,807), topography modifications, magneto-optic media, photochromism, bubble formation, and ablative/melt media. A review of such medias can be found in Wen-yaung Lee, Journal of Vacuum Technology A3(3), pp 640-645 (1985). The ablative/melt media is the most commonly practiced form of WORM system The data is typically stored in the form of small pits or depressions which are created through local heating by the laser radiation of a thin active layer (most commonly of a metal or a metal alloy) to a threshold level in which the material in the active layer begins to melt and/or ablate from the surface of the disc. Ablation in this application means modification of the surface resulting in structural changes that are optically detectable.
Examples of such media typically incorporate tellurium-based alloys as part of the active layer due to its relatively low melting/ablating point. By melting and/or ablating the tellurium alloy in a localized area, a void is produced that exposes the underlying substrate. The void can be optically detected using either of at least two structure configurations.
In the first type, a second substrate is suitably affixed to the first. It is spaced from the first tellurium-coated substrate, so as to provide an air space between the two substrates. Since the tellurium alloy has a smooth reflective surface, the radiation is suitably reflected back off this surface After void formation, light traverses through the first transparent substrate and is internally reflected and lost in the air space. Thus, the unaltered surface appears reflective and the voids appear dark. This configuration is often called an Air-Sandwich Write-Once Disc (Kenny U.S. Pat. No. 4,074,282 and Lehureau et al. U.S. Pat. No. 4,308,545).
In a second configuration, a low reflective tellurium alloy is deposited on a transparent dielectric which in turn overlies a metallized substrate (e.g. aluminum). This configuration is commonly referred to as an anti-reflection tri-layer structure. The thickness of the dielectric is carefully chosen to produce destructive interference of the incident light used for reading the information from the disc. Thus, before writing, the medium absorbs nearly all the radiation of the laser reading source. Where the voids are formed, the absence of the tellurium alloy precludes destructive interference of the laser radiation, and light is now reflected from the underlying metal surface.
Both of these configurations represent the digital information in the form of differential absorbing and reflecting areas within the disc and require complex design considerations to meet acceptable signal-to-noise standards.
Despite an extensive research effort, systems which are based on tellurium and alloys thereof exhibit many characteristics which are highly undesirable for commercial WORM systems and cannot be used for recordable compact disc memory media. First of all, alloys of tellurium are typically very unstable upon exposure to oxygen and/or moisture, thus making expensive hermetically sealed disc structures necessary in order to ensure a minimum archivability of the data for at least 10 years (Mashita et al. U.S. Pat. No. 4,433,340). Secondly, the use f tellurium in the fabrication process requires great care due to the inherent toxicity of tellurium and alloys thereof. Additionally, the low light-reflective contrast ratios between the unaltered tellurium film and the laser created voids lead to significant errors in data storage and retrieval. Ideally, contrasts of greater than 5 to 1 are required to give accurate and reliable forms of media. Also, tellurium-based systems typically exhibit about 35% to 40% reflectivity which will not meet standards for recordable compact disc memory media.
Several criteria considerations made during the selection and design of an active layer are strongly dependent on optimizing these contrast ratios within the media. For example, because speed is an extremely important consideration in recording and retrieving data, it is highly desirable that the active thin metal alloy reflective layer, in an ablative/melt type of WORM or recordable compact disc memory media, be of a suitable composition so as to enable the laser to rapidly form the pit or depression as well as to minimize the laser energy required. Further, it is important that the active/reflective metal surface area, surrounding the pit thus formed, remain relatively unaffected during data retrieval: as any disruption of this surface will result in degradation of the optical signal during the reading process, thereby leading to an increase in the bit-error-rate (BER) and decreasing the signal-to-noise ratio.
In all of the above referenced applications (ROM, WORM and recordable compact disc memory), it is extremely important that the active reflective surface be uniform and able to provide sharp, clear contrasts between the information spots and surrounding surface in order that the resulting signal-to-noise ratio may be maximized. Further, in order to ensure long term retrievability of the encoded information, it is desirable that the active reflective surface material be highly resistant to corrosion and oxidation, which may result in degradation of the signal. It is important that the reflective material be firmly bonded to the supporting substrate and be dimensionally and environmentally stable. It is also advantageous that the active surface be highly reflective so that a secondary reflecting layer is not required. This results in a lower cost optical storage medium.